This invention relates to telephone systems and, in particular, to providing full duplex operation for more than two parties to a telephone call. As used herein, “telephone” includes cellular telephones.
There are two kinds of echo in a telephone system, an acoustic-echo between an earphone or a speaker and a microphone and an electrical echo occurring in the switched network for routing a call between stations. In a handset, acoustic echo is typically not much of a problem. In speaker phones, where several people huddle around a microphone and loudspeaker, acoustic feedback is much more of a problem. Hybrid circuits (two-wire to four-wire transformers) located at terminal exchanges or in remote subscriber stages of a fixed network are the principal sources of electrical echo, also known as line echo.
Filtering a voice signal to eliminate either or both kinds of echo is known in the art. Devices known as complementary comb filters have been used to eliminate echoes by having the signal to a speaker filtered through the pass bands of a first comb filter, thereby falling within the stop bands of a second, complementary comb filter coupled to a microphone. Matching, rather than complementary, comb filters can be used in the line out and line in channels of a telephone if one also uses a frequency shift; see U.S. Pat. No. 4,748,663 (Phillips et al.). Frequency shifting is undesirable because of the effect on the quality of the voice signal.
Complementary filters work well when there are only two parties to a call. For a conference call, at least one of the phones must be in half-duplex mode. Switching unobtrusively between half duplex and full duplex among the parties is difficult at best and invariably one of the parties is blocked momentarily while speaking.
One could provide three complementary comb filters (each containing one pass band and two stop bands) for a three party call but the cost of such a system is prohibitive. Further, the reduction in spectral power would significantly reduce the quality of the audio signal.
Even with well designed band pass filters, a comb filter necessarily reduces the power and spectral content of speech. For example, an amplitude peak may happen to fall within the stop band of a comb filter, substantially changing the sound characteristic of a person's voice. When fricatives fall within a stop band, intelligibility can be significantly reduced. Amplification is not a cure if the filters do not match the spectral response of an person's voice.
In other applications, e.g. automotive cellular telephones, certain sounds are noises characteristic of the vehicle or environment rather than the driver and it would be desirable to have a stop band match the dominant frequency of the noise. Again, comb filters of the prior art cannot remove such noise except by chance.
In view of the foregoing, it is therefore an object of the invention to provide an improved band pass filter system in which each band is chosen individually depending upon the spectral content of the applied signal.
Another object of the invention is to provide a band pass filter system in which full duplex communication is optimized for the voice characteristics of the respective speakers.
A further object of the invention is to provide a band pass filter system in which the pass bands can be allocated among three parties in a phone call.
Another object of the invention is to provide a band pass filter system that can emulate a comb filter initially and then adapt to the spectral content of each channel.